linux-kernel - Re: [PATCH 2/7] rcu: limit PREEMPT

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20241017080439.9od9eoBO@linutronix.de>
Date: Thu, 17 Oct 2024 10:04:39 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Ankur Arora <ankur.a.arora@...cle.com>
Cc: "Paul E. McKenney" <paulmck@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org,
	tglx@...utronix.de, mingo@...nel.org, juri.lelli@...hat.com,
	vincent.guittot@...aro.org, dietmar.eggemann@....com,
	rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
	vschneid@...hat.com, frederic@...nel.org, efault@....de
Subject: Re: [PATCH 2/7] rcu: limit PREEMPT_RCU configurations

On 2024-10-15 15:13:46 [-0700], Ankur Arora wrote:
> Sebastian Andrzej Siewior <bigeasy@...utronix.de> writes:
> 
> >>
> >> As for PREEMPT_LAZY, you seem to be suggesting a more intrusive change
> >> than just keeping non-preemptible RCU when the Kconfig options are
> >> consistent with this being expected.  If this is the case, what are the
> >> benefits of this more-intrusive change?
> >
> > As far as I understand you are only concerned about PREEMPT_LAZY and
> > everything else (PREEMPT_LAZY + PREEMPT_DYNAMIC or PREEMPT_DYNAMIC
> > without PREEMPT_LAZY) is fine.
> > In the PREEMPT_LAZY + !PREEMPT_DYNAMIC the suggested change
> >
> > | config PREEMPT_RCU
> > | 	bool
> > | 	default y if (PREEMPT || PREEMPT_RT || PREEMPT_DYNAMIC)
> > | 	select TREE_RCU
> > | 	help
> >
> > would disable PREEMPT_RCU while the default model is PREEMPT. You argue
> 
> With PREEMPT_LAZY=y, PREEMPT_DYNAMIC=n, isn't the default model
> PREEMPT_LAZY, which has PREEMPTION=y, but PREEMPT=n?

Correct.

> > that only people on small embedded would do such a thing and they would
> > like to safe additional memory.
> >
> > I don't think this is always the case because the "preemptible" users
> > would also get this and this is an unexpected change for them.
> 
> Can you clarify this? The intent with lazy is to be preemptible but
> preempt less often. In that it is meant to be quite different from
> CONFIG_PREEMPT.

A wake up with PREEMPT may not always lead to a preemption but will lead
to preemption once the time slice is up. With LAZY this changes to the
point that a preemption point will be delayed to the sched tick. Tasks
which are not based on the fail class (SCHED_DL, FIFO, …) will receive a
wake up right away.

If in the long term NONE and VOL goes away you could argue that everyone
using LAZY + !DYNAMIC is one of those. If additionally PREEMPT goes away
then you can not. Therefore I would prefer to have the RCU model to be
selectable rather than forced. While !PREEMPT_RCU may save memory, it
also disable preemption for the read section. 

> Ankur

Sebastian